Ground rolling bale forming machines are well-known in the art as evidenced by U.S. Pat. Nos. 3,110,145, 3,680,296, 3,797,215, 3,815,344 and 4,183,197. In these patents, the material picked up by the pick-up means is rolled along the ground and a round bale is formed by the action of a lower run of an endless flexible means such as belts or the like. This lower run is expandable upwardly in response to the growing diameter of the bale and is trained about members spaced a fixed distance apart to accommodate therebetween the diameter of the completed bale. In the forming of the bale, therefore, only a relatively small portion of its peripheral surface is acted upon by the lower run whereby to appreciably reduce the efficiency of the flexible means to roll and compact the forming bale. These disadvantages of the prior art are eliminated in the bale forming machine of the present invention.